Formaldehyde LIF detection with background subtraction around single igniting GTL diesel droplets

•Images of first stage GTL diesel fuel droplet ignition.•New technique for background subtraction.•Ignition delay time based on laser-induced formaldehyde fluorescence. Formaldehyde is an important combustion intermediate, which is often used for the analysis of turbulent flames, internal combustion engines or droplet/spray ignition processes. Due to its high concentration in hydrocarbon fuel combustion, laser induced fluorescence (LIF) of formaldehyde can easily be excited (e.g. 339, 343, 353 and 355nm) and measured (360–550nm). However, it is often superimposed by background emission from other combustion intermediates as, e.g. polycyclic aromatic hydrocarbons (PAHs). We present formaldehyde LIF excitation spectra obtained with a XeF excimer laser in combination with a spectrometer and an ICCD camera in dependence of pressure (50mbar to 20bar). For formaldehyde LIF and background excitation, we selected the wavelengths 353.373 and 353.386nm, respectively. LIF measurements at the background wavelength contain a rising formaldehyde contribution with increasing pressure. We present a novel method for the calculation of the real background. This method is applied to the ignition of single gas-to-liquid (GTL) diesel droplets. However, the method can also be applied to other fuels. In the case of a GTL diesel ignition experiment (628K, 3bar) the real background amounted to only 30% of the emission which was measured at the background excitation wavelength (353.386nm).